A fuel cell is a kind of electric power supply capable of generating electric energy by electrochemically oxidizing a fuel such as hydrogen or methanol, and an intense interest has been shown towards the fuel cell, as a clean energy supply source, recently. Particularly, it is expected that a polymer electrolyte fuel cell is widely used as a distributed power generation facility of comparatively small scale, and a power generator of mobile bodies such as automobile and marine vessel, because of such low standard operation temperature as about 100° C. and high energy density. Also, an intense interest has been shown towards the polymer electrolyte fuel cell as a power supply of portable mobile equipment and a portable device, and it is expected to install the polymer electrolyte fuel cell in a cellular phone and a personal computer in place of a secondary cell such as nickel-hydrogen cell or lithium ion cell.
In the fuel cell, anode and cathode in which the reaction capable of generating electricity occurs, and a polymer electrolyte membrane being used as a proton conductor between the anode and the cathode usually constitute a membrane electrode assembly (hereinafter sometimes abbreviated to MEA) and a cell comprising separators and MEA sandwiched between the separators is formed as a unit. The polymer electrolyte membrane is mainly composed of the polymer electrolyte material. The polymer electrolyte material is also used for a binder of an electrocatalyst layer or the like.
As a polymer electrolyte material, aromatic polyetherether ketone, aromatic polyetherketone and aromatic polyethersulfone have been actively investigated in point of heat resistance and chemical stability.
Also, in the sulfonated compound (for example, patent documents 1 and 2) of an aromatic polyetherketone (hereinafter, sometimes abbreviated to PES) (examples thereof include VICTREX PEEK-HT, manufactured by VICTREX PLC), there was a problem that because its crystallinity is high, a polymer having the composition of low density of a sulfonic acid group becomes insoluble in a solvent, resulting in poor processability because of a remained crystal moiety. To the contrary, when the density of the sulfonic acid group is increased so as to enhance processability, the polymer is not crystalline and drastically swells in water, and therefore, purification of the polymer becomes very difficult and production of the polymer was not easy.
As a method of controlling an amount of the sulfonic acid group, an example, in which a monomer having a sulfonic acid group introduced is polymerized to form sulfonated aromatic polyethersulfone in which an amount of a sulfonic acid group is controlled, is reported in an aromatic polyethersulfone type (for example, patent document 3). However, also in this method, a problem that a membrane prepared at elevated temperature and at high humidity swells is not solved, and this tendency is remarkable particularly in a fuel solution such as methanol or in composition in which a sulfonic acid group density is high, and in such a polymer electrolyte membrane which is inferior in resistance to hot water and resistance to hot methanol, it was difficult to adequately inhibit fuel crossover such as methanol or the like and to impart mechanical strength for enduring cycling of swelling and drying.
As described above, the polymer electrolyte material according to prior art is insufficient as measures for improving economic efficiency, processability, proton conductivity, fuel crossover, mechanical strength and therefore long-term durability, and there has never been obtained an industrially useful polymer electrolyte material for a fuel cell.
As an invention to solve these problems, in patent document 4, there is proposed a method in which a polymer having a crystallization power is converted to a solution by introduction of a protective group (hydrolytic group for imparting solubility), a film is formed from the solution, and then deprotection (hydrolysis) is carried out, and it is said that by evaluating mechanical characteristics and improving a relation between a chemical structure and resistance to hot water, resistance to hot methanol and processability, an electrolyte membrane, which is excellent in proton conductivity, fuel barrier properties, mechanical strength, resistance to hot water, resistance to hot methanol, processability and chemical stability, can be provided. However, further improvement has been desired.
Further, in patent document 5, there is proposed an electrolyte membrane which is excellent in conductivity and durability by reinforcing with a porous film. However, in patent document 5, since an assembled membrane in which a fluorine-type electrolyte membrane is packed in a fluorine-type fine porous membrane is disclosed, and an exchange capacity of the electrolyte membrane used in an example is 1.25 meq/g, proton conductivity as an assembled polymer electrolyte membrane was insufficient, and moreover since a constituent polymer is a fluorine-type, a hydrogen gas easily permeates through the membrane and therefore durability in an open circuit state in operating a fuel cell using the electrolyte membrane was insufficient.